There might be a scientific amount to chop off, 3mm seems right, but I find crimping and sleeving to be a skill that is based on experience.
I do like this image though:
A key tip is to look at the copper on the right side of the conductor crimp. Notice how NONE of it goes into the mating section.
The little bit of insulator on the right side of the insulator crimp is key as well, if you don't have any there redo your crimp.
Finally, the crimping tools are not DITIGAL in that you dont just clamp down on them and expect it to be precise. They are analog and require experience to know just how far you need to go down and it changes with your wire, wing size, die, and heaven forbid you use anything but MOLEX brand Mini-Fit jr. connectors.
Peace!